This invention relates to the production of foamable resins, and particularly to a process for the production of foamable vinyl aromatic polymer particles as well as to foamed articles made from them.
Foamable polyvinyl aromatic resin particles, for instance beads, are an important industrial commodity. For example, foamable polystyrene beads produced by polymerizing styrene in an aqueous suspension system in the present of a volatile hydrocarbon blowing agent are commonly used in the production of foamed polystyrene by a process in which they are expanded by the action of heat so as to produce a "prefoam" of foamed beads and these foamed beads are placed in a suitable mold and heated by steam in a molding operation. Here, further expansion and consolidation of the beads takes place and an article conforming to the shape of the mold is produced.
It has been found that the best results are obtained when the beads used to make an article are all of approximately the same size and that difference sizes of bead are required for different classes of article. Thus, relatively large beads are usually best for making blocks intended to be cut up into sheets for insulation purposes, somewhat smaller beads are usually employed for making ceiling tiles, smaller ones still for making packages for electronic or other delicate equipment and the smallest useful size of beads is usually employed for making disposable drinking cups or other small containers. It is, therefore, often found that there is a greater demand for one size of bead than for another and this requires elaborate control of the suspension polymerization process by which the beads are made so as to produce the different sizes in the required proportion. It may even be necessary to discard some beads for which no market exists and such waste is economically undesirable. Moreover, sieving the foamable beads to separate the different grades can in some way result in a tendency for the "prefoamed" beads to clump together in the prefoaming apparatus and block it and special measures are needed to counteract this tendency.
More recently processes have been developed to produce foamable beads by impregnating or incorporating the blowing agent in the beads after they are polymerized by suspending the beads in an aqueous suspension and incorporating the blowing agent into the beads under pressure following by cooling and drying the beads so as to remove residual surface and occluded water under conditions that will remove the residual water without removing the volatile blowing agents. Here, both processes, i.e., the polymerizing of beads in the presence of the blowing agent or impregnating the beads after polymerization have the same problem of economically drying such foamable beads so as to remove the residual water yet retaining substantially all of the incorporated blowing agent.
Conventionally such foamable beads have been separated by centrifuging the beads or screening the beads from the aqueous suspension medium. Centrifuged beads typically contain about 1% residual water and about 7% blowing agent, e.g pentane. The beads are air dried to remove surface water and occluded water. The beads are conventionally dried in a continuous rotary air drier at temperatures of about 120.degree. to 125.degree. F. (49.degree. to 52.degree. C.) as disclosed in U.S. Pat. No. 3,060,138.
It has been discovered that such foamable beads can be dried removing residual water under vacuum conditions at normal temperatures in relatively short time periods, e.g. ten to twenty minutes yet retaining substantially all the volatile blowing agent incorporated in the bead. This ability of this process to remove residual water at high rates without drastically reducing the blowing agent content was totally unexpected considering the volatility of the blowing agents. Beyond the superior drying rates the process provides a dried foamable bead that has a superior fine cell structure not attainable with conventional drying.